FIG. 8 is a cross-sectional view showing a prior art semiconductor laser producing visible light, for example, as recited in "InGaAlP Transverse Stabilized Visible Laser Diodes Fabricated by MOCVD Selective Growth" by M. Ishikawa, Y. Ohba, Y. Watanabe, H. Nagasaka and H. Sugawara, Extended Abstracts of the 18th Conference on Solid State Devices and Materials, Tokyo, 1986, pp 153-156. In FIG. 8, numeral 13 designates an n type GaAs substrate. An n type GaAs layer 14 is deposited on the substrate 13, an n type AlInP lower cladding layer 15 is deposited on the n type GaAs layer 14, a GaInP active layer 16 is deposited on the lower cladding layer 15, and a p type AlInP upper cladding layer 17 including a stripe-shaped ridge is deposited on the active layer 16. A p type GaInP buffer layer 19 is deposited on the ridge part of the upper cladding layer 17, and an n type GaAs blocking layer 18 is deposited on regions other than the ridge part of the upper cladding layer 17 and on the side face of the ridge by means of selective growth. A p type GaAs contact layer 20 is deposited on the buffer layer 19 and on the blocking layer 18. A p side electrode 22 is deposited on the contact layer 20, and an n side electrode 21 is deposited on the rear surface of a substrate 1.
When a forward bias is applied to the pn junction between the n type GaAs substrate 13 and the contact layer 20, current is confined by the blocking layer 18, and is injected from the ridge into the active layer 16. These injected carriers ar confined in the active layer 16 by a hetero junction, and recombine to emit light. Furthermore, a difference in refractive index is produced in the horizontal direction of the active layer 16 by the light absorption and the current confinement by the blocking layer 18, and broadening of light in the lateral direction is limited. Light guided by such a waveguide causes laser oscillation in a Fabry-Perot resonator constituted by the end facets facing each other which are perpendicular to the direction of the length direction of the stripe-shaped ridge.
The prior art semiconductor laser is configured as described above, and therefore, to reduce leakage current and produce a difference in refractive index, it is required to perform etching so as to make a layer thickness d of the upper cladding layer 17 outside the ridge as thin as 0.2-0.3 micron. It is difficult to form the ridge with good reproducibility, and the laser characteristics are not uniform due to variations in the above-mentioned layer thickness d, resulting in a reduction in yield and poor reproducibility. Also, restricted by photolithography and etching, a ridge width W cannot be narrowed to about 1-2 microns, and therefore the laser light beam has a slender elliptic cross-section. Furthermore, the semiconductor laser shown in the conventional example has problems in that three separate crystal growth steps are required, processes in fabricating the laser are complicated and so on.